The goal of low-noise oscillator design is to produce a signal having a high signal-to-noise ratio by maximizing the amplitude of the signal and minimizing the circuit noise level. Maximizing the energy stored in a resonator results in improved phase-noise performance because a high signal amplitude is available to the resonator load. When the energy stored in a resonator is maximized, the power dissipated in it is also at maximum.
One approach to maximizing the power dissipated in a resonator is to deliver constant power to it by limiting the voltage or current delivered to it. However, power-limited resonators such as crystal resonators can have large tolerances in their equivalent resistance at resonance. For example, the resistance of a crystal resonator might vary ±40% about its nominal resistance. When constant voltage or constant current is delivered to such a resonator, the power dissipated in the resonator will vary by that resistance tolerance.
What is desired is an oscillator capable of maximizing signal-to-noise ratio with a resonator having a large resistance tolerance.